I was running a simulation the other day using a virtual bolt.  When I checked the tension in the bolt (M10 x 1.5 torqued to 30 ft-lbs), I expected to see around 4500 lbs, but it was around 3700 lbs.  I did some digging into the SolidWorks help files and discovered that for a bolt with a nut SW uses the formula F=T/(K*D) whereas for a bolt without a nut (as in my simulation example) SW uses the formula F=T/(K*D*1.2).  This gives the bolt without a nut 83% of the tension of a bolt with a nut.  Where does the 1.2 factor come from (it is unexplained in the help file)?  Am I missing something?  Is bolting into a nut fundamentally different than bolting into a threaded plate?